1. Field of the Invention
The invention relates to a contactless inductively coupled power transfer (ICPT) system.
2. Description of the Related Art
A wireless power transfer system has been used to replace some conventional contact conductive power transfer systems because the wireless power transfer system is capable of transferring power in a contactless manner and thus may be used in situations where wire connections are unsuitable.
In U.S. Patent Application Publication No. 20070109708, a conventional wireless power transfer system is disclosed. The wireless power transfer system includes a supplying device and a receiving device, each of which includes a winding used for transferring electric power. In general, the receiving device of the wireless power transfer system may include a conventional LC resonant circuit. Inductance and capacitance of the conventional LC resonant circuit are adjustable, such that a level of an output voltage of the receiving device and a resonant frequency of the conventional LC resonant circuit thereof may be controlled.
When the conventional LC resonant circuit implemented using a series connection of an inductor and a capacitor is resonating, resultant impedance of the series connection of the inductor and the capacitor is substantially zero. Hence, the conventional LC resonant circuit may be regarded as an ideal voltage source but with a voltage gain equal to or smaller than one. When the conventional LC resonant circuit implemented using a parallel connection of an inductor and a capacitor is resonating, the voltage gain thereof may be greater than one, but resultant impedance of the parallel connection of the inductor and the capacitor is substantially infinite. Such conventional LC resonant circuit may be regarded as an ideal current source.
The practical demand is an ideal voltage source with a characteristic of variable voltage gain. The reason for this is that when the supplying device and the receiving device are relatively distant from each other, coupling therebetween is relatively weak, and thus an induced voltage of the receiving device may drop to a level that is insufficient for power supply and may not meet requirements of some loads.
Furthermore, when observed from the supplying device, the receiving device often has input impedance. The input impedance when observed from the supplying device may vary according to different factors, such as coupling, load condition of the receiving device, tuning condition of the receiving device, etc. Variation of the input impedance may change a current flowing through the winding of the supplying device. Since the winding of the supplying device is adapted to generate an electromagnetic field in response to the current flowing through the winding, the change of the current may result in instability of power supply at the receiving device.